When information printed on a medium such as paper is keep secret, the paper or the like is generally contained in a particular storage location, a book storeroom with a key, or the like. However, it is inconvenient to accommodate secret documents frequently created every day in a particular storage location, a book storeroom with a key, or the like one by one.
A method of enciphering information and printing the enciphered information on paper or the like is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 1-147750. This makes it possible to store printed paper without accommodating the paper in a particular storage location or the like. However, the cryptography described in this publication is to only convert document data (plaintext data) into ciphertext data in accordance with a predetermined encryption table. Accordingly, the rule of encryption may be easily analyzed by a third person so that the ciphertext data is easily deciphered. Consequently, it is difficult to maintain secrecy about information even by the method described in this publication.